Loudness control with noise detection and loudness drop detection

Abstract

Loudness control systems or methods may normalize audio signals to a predetermined loudness level. If the audio signal includes moderate background noise, then the background noise may also be normalized to the target loudness level. Noise signals may be detected using content-versus-noise classification, and a loudness control system or method may be adjusted based on the detection of noise. Noise signals may be detected by signal analysis in the frequency domain or in the time domain. Loudness control systems may also produce undesirable audio effects when content shifts from a high overall loudness level to a lower overall loudness level. Such loudness drops may be detected, and the loudness control system may be adjusted to minimize the undesirable effects during the transition between loudness levels.

What is claimed is: 
     
       1. A loudness control system configured to process an audio signal, the loudness control system comprising:
 a loudness measurement circuit configured to generate a short-term loudness estimate of the audio signal, wherein the audio signal includes content and noise; 
 a noise detection circuit configured to produce a content-versus-noise classification of the audio signal indicating whether the audio signal is more likely to be content or more likely to be noise; 
 a temporal smoothing circuit configured to adjust at least one smoothing factor based on the content-versus-noise classification to control a speed at which a gain correction circuit can increase a time-varying gain, and generate a long-term loudness estimate of the audio signal based on the short-term loudness estimate using the at least one smoothing factor; and 
 the gain correction circuit configured to apply the time-varying gain to the audio signal based on the long-term loudness estimate. 
 
     
     
       2. The loudness control system of  claim 1 , wherein:
 the noise detection circuit is configured to use frequency domain noise detection to produce the content-versus-noise classification. 
 
     
     
       3. The loudness control system of  claim 1 , wherein:
 the noise detection circuit is configured to use time domain noise detection to produce the content-versus-noise classification. 
 
     
     
       4. The loudness control system of  claim 1 , wherein:
 the at least one smoothing factor includes a release smoothing factor. 
 
     
     
       5. The loudness control system of  claim 1  further comprising:
 a loudness drop detection circuit configured to generate a loudness drop detection value; 
 wherein the temporal smoothing circuit is further configured to adjust the at least one smoothing factor based on the loudness drop detection value. 
 
     
     
       6. A loudness control system configured to process an audio signal, the loudness control system comprising:
 a loudness measurement circuit configured to generate a short-term loudness estimate of the audio signal, wherein the audio signal includes content and noise; 
 a loudness drop detection circuit configured to generate a loudness drop detection value based in part on a slow smoothed loudness estimate of the loudness estimate of the audio signal and fast smoothed loudness estimate of the loudness estimate of the audio signal; 
 a temporal smoothing circuit configured to adjust at least one smoothing factor based on the loudness drop detection value to control a speed at which a gain correction circuit can increase a time-varying gain, and generate a long-term loudness estimate of the audio signal based on the short-term loudness estimate using the at least one smoothing factor; and 
 the gain correction circuit configured to apply the time-varying gain to the audio signal based on the long-term loudness estimate. 
 
     
     
       7. The loudness control system of  claim 6 , wherein:
 the at least one smoothing factor includes a release smoothing factor. 
 
     
     
       8. The loudness control system of  claim 6  further comprising:
 a noise detection circuit configured to produce a content-versus-noise classification of the audio signal; 
 wherein the temporal smoothing circuit is further configured to adjust the at least one smoothing factor based on the content-versus-noise classification. 
 
     
     
       9. A method for processing an audio signal, performed by a loudness control system, the method comprising:
 generating a short-term loudness estimate of the audio signal, wherein the audio signal includes content and noise; 
 producing a content-versus-noise classification of the audio signal indicating whether the audio signal is more likely to be content or more likely to be noise; 
 adjusting at least one smoothing factor based on the content-versus-noise classification to control a speed at which a gain correction circuit can increase a time-varying gain, and generating a long-term loudness estimate of the audio signal based on the short-term loudness estimate using the at least one smoothing factor; and 
 applying the time-varying gain to the audio signal based on the long-term loudness estimate. 
 
     
     
       10. The method of  claim 9 , wherein:
 the producing the content-versus-noise classification uses frequency domain noise detection. 
 
     
     
       11. The method of  claim 9 , wherein:
 the producing the content-versus-noise classification uses time domain noise detection. 
 
     
     
       12. The method of  claim 9 , wherein:
 the at least one smoothing factor includes a release smoothing factor. 
 
     
     
       13. The method of  claim 9  further comprising:
 generating a loudness drop detection value; and 
 adjusting the at least one smoothing factor based on the loudness drop detection value. 
 
     
     
       14. A method for processing an audio signal, performed by a loudness control system, the method comprising:
 generating a short-term loudness estimate of the audio signal, wherein the audio signal includes content and noise; 
 generating a loudness drop detection value based in part on a slow smoothed loudness estimate of the loudness estimate of the audio signal and fast smoothed loudness estimate of the loudness estimate of the audio signal; 
 adjusting at least one smoothing factor based on the loudness drop detection value to control a speed at which a gain correction circuit can increase a time-varying gain, and generating a long-term loudness estimate of the audio signal based on the short-term loudness estimate using the at least one smoothing factor; and 
 applying the time-varying gain to the audio signal based on the long-term loudness estimate. 
 
     
     
       15. The method of  claim 14 , wherein:
 the at least one smoothing factor includes a release smoothing factor. 
 
     
     
       16. The method of  claim 14  further comprising:
 producing a content-versus-noise classification of the audio signal; and 
 adjusting the at least one smoothing factor based on the content-versus-noise classification.